In many machines, such as machine tools, it is important to precisely measure the distance travelled by one component relative to another. For example, in a machine tool, it is often important to precisely measure the distance travelled by the spindle relative to the base on which the workpiece is mounted to permit machining of the workpiece within predetermined tolerance limits. Other machine tools use a plurality of extensible legs to move the cutting tool and workpiece relative to one another. This type of machine requires that the amount of extension or contraction of each leg be precisely monitored to accurately cut the workpiece.
A variety of measurement devices have been incorporated into various machines to monitor the distances traveled and to provide an output signal to a controller which, in turn, precisely controls the relative distances moved between the cutting tool and the workpiece.
One type of measurement device uses a stationary grid having a pattern of equally spaced marks along the grid. A sensing head is mounted to a moveable component of the machine and moves along the grid sensing the indicator marks to provide a signal indicative of the moving component's position relative to the stationary grid.
More precise measurement may be obtained by using a laser interferometer. The movement of one component relative to another component is measured by mounting a mirror on each component. A laser light source then generates a laser beam that is split into two components by the first mirror. One component is immediately reflected back towards a photo detector while the other component is reflected to the second mirror disposed on the other moveable component. This second mirror reflects the light back to the first mirror where the two laser beam components are recombined and reflected to the photo detector. The photo detector reads the fringes resulting from interference between the two components of the laser beam as the second mirror is moved relative to the first mirror and the two laser beam components move in and out of phase. The fringes are indicative of changes in the distance between the two mirrors, and by counting the fringes, the relative movement between the components can be determined.
In other words, the laser interferometer measures relative displacement by causing two beams of light to interfere. The light beams are created when a single monochromatic beam is split into two separate light beams. Those beams are caused to follow different paths to separate mirrors where they are reflected back towards a photo detector and recombined.
The intensity of the combined beams depends on the phase difference between those beams. When they are in phase, their intensities add, but when they are 180.degree. out of phase they subtract. Thus, if one of the mirrors moves by an amount equal to one quarter wavelength or 90.degree., the roundtrip difference is 180.degree.. Therefore, the recombined beam will undergo a complete phase change as one of the mirrors moves a distance of one half wavelength with respect to the other mirror. In the typical machine application, one mirror remains stationary with respect to the beam splitter and provides a reference path. Accordingly, all apparent interference (fringes) can be assumed as caused by displacement of the other mirror.
One problem that arises with the use of laser interferometers to measure relative distances is that the wavelength of light is affected by changes in air temperature, pressure, and humidity. The laser interferometer may therefore indicate relative movement when no movement has occurred. This complication is compounded by the fact that, in many machine applications, the minimum measurement path length can be fairly large. In other words, the configuration of the machine makes it impossible to move both mirrors into proximity with the beam splitter to effectively calibrate the interferometer at the same beginning reference point prior to each use of the machine. If this were possible, then automatic compensators could be used to approximate the differences in wavelength as a function of the changing variables, such as temperature, pressure, and humidity. Also, if the minimum distance between the moving mirror and the beam splitter, commonly called the deadpath distance, were precisely known, automatic compensators could also be used to estimate the changes in wavelength. However, for many applications, automatic compensators do not provide adequate accuracy in measurement to the extent achievable by controlling the variables that effect the wavelength of light.
Thus, it would be advantageous to provide an economical way to control the variables that affect the light wavelength to permit accurate measurement of distances between components that move relative to one another.